Earth Hour comes on the last Saturday of March, so for 2011 this means March 26, between 8:30 and 9:30 p.m.

It was designed to inspire people around the world to conserve energy, but it has often been used as an excuse for an end-of-winter party in climates where winter means snow, ice, weather fatigue and going out like a raging lion or a gentle lamb.

This year, 131 countries and territories are registered to take part in Earth Hour, the most ever.

"It is only through the collective action of business, organizations, individuals, communities and governments that we will be able to affect change on the scale required to address the environmental challenges we face," said Andy Ridley, co-founder and executive director of Earth Hour.

The Quebec legislature, during and after Earth Hour 2009. (Jacques Boissinot/Canadian Press)

It started five years ago in Sydney, Australia, the brainchild of the World Wildlife Fund (WWF). More than two million people participated, resulting in a 10 per cent reduction in demand on Sydney's electrical grid.

It's gone global now, with nearly a billion people expected to take part this Saturday from Australia to Beijing's Forbidden City to the Leaning Tower of Pisa to the great sprawl that is Canada. Yes, the lights will go out on Niagara Falls in Canada and the U.S.

The goal is to cut electricity consumption by five per cent for an hour. Sounds great, and makes people feel good, but Bob McDonald of CBC Radio's Quirks says what Earth Hour achieves really is a drop in the bucket.

“According to the Intergovernmental Panel on Climate Change, there needs to be a 50 per cent to 80 per cent reduction in emissions to turn the current warming trend around," McDonald wrote. "That's a long way from a big party held one Saturday night a year.”

Last year, 126 countries participated in Earth Hour, with an estimated 80 to 90 million joining in as the lights went out on landmarks such as Mount Rushmore and the Empire State Building.

Results were mixed in Canada last year. Toronto saved 900 megawatt-hours, which meant 8.7 per cent when measured against a typical March Saturday night. The worst result in the world, however, came from Calgary where power consumption actually went up 3.6 per cent during Earth Hour because temperatures had dropped 12 degrees Celsius from the previous Saturday.

This year, the Earth Hour motto from WWF is “Go beyond the hour.”